Interest in AlGaN ultraviolet light-emitting diodes (UVLEDs) is driven by applications such as water purification, bio-agent detection, and data storage. See A. Khan et al., Nat. Photonics 2, 77 (2008). Many challenges remain to achieve higher device efficiencies, including a better understanding of the variables that influence the polarization anisotropy of luminescence of AlGaN quantum wells (QWs) grown with c-plane orientation. The optical polarization is important because it determines the emission patterns within the AlGaN layers and therefore has a profound impact on the extraction efficiency of UVLEDs. Ideally, light emitted from AlGaN quantum wells would be completely polarized parallel to the QW plane because such light propagates predominately perpendicular to the QW plane, and into the typical and more efficient light escape cones. This is favored over light propagation along the growth plane which requires multiple and lossy bounces before extraction. Parallel polarized emission occurs in InGaN QWs, and results in extremely high light extraction efficiencies. See M. R. Krames et al., J. Disp. Technol. 3, 160 (2007). The optical polarization of AlGaN QWs, on the other hand, exhibits anisotropy and under certain conditions can also emit a percentage of the light polarized perpendicular to the QW plane. This light polarization propagates parallel to the QW plane and, as mentioned before, is more difficult to extract.
The strength and fraction of the two optical polarizations from the AlGaN QWs are determined by the complex structure of the valence band. The structure of the valence band is determined by many variables. Gaining a deeper understanding of these variables, and how they influence the valence band will result in better device designs for higher light extraction efficiency.
One major, and well-known, variable contributing to optical polarization is the Al composition of the AlGaN QW layers. Assuming all other variables are constant, as the Al mole fraction within the AlGaN alloy increases, the crystal-field split-off (CH) valence band eventually becomes the highest energy valence band (ground state), due to a negative crystal-field splitting, and the polarization shifts. See M. Suzuki et al., Phys. Rev. B 52, 8132 (1995); and W. W. Chow and M. Kneissl, J. Appl. Phys. 98, 114502 (2005). This energy cross-over of ground state valence bands occurs at Al compositions of ˜0.6-0.8. See R. G. Banal et al., Phys. Rev. B 79, 121308 (2009); T. K. Sharma et al., Phys. Rev. B 84, 035305 (2011); and J. Zhang et al., Appl. Phys. Lett. 97, 111105 (2010). The light emission from this high-Al-content AlGaN layer is polarized perpendicular to the QW plane, and is, for the most part, trapped and lost within the AlGaN light-emitting diode (LED) layers because it is outside the typical light extraction cones for planar structures that are perpendicular to the QW plane. See W. W. Chow and M. Kneissl, J. Appl. Phys. 98, 114502 (2005); K. B. Nam et al., Appl. Phys. Lett. 84, 5264 (2004); J. Shakya et al., Appl. Phys. Lett. 86, 091107 (2005); H. Kawanishi et al., Appl. Phys. Lett. 89, 081121 (2006); Y. Taniyasu et al., Appl. Phys. Lett. 90, 261911 (2007); and T. Kolbe et al., Appl. Phys. Lett. 97, 171105 (2010). As a result, the light is trapped within the UVLED and absorbed by metal and lower bandgap layers (such as p-GaN).
Another factor that influences optical polarization is the strain of the QW relative to underlying substrate or template layers. The strain of the AlGaN layer can alter the critical Al composition where the optical polarization switches from parallel to perpendicular to the QW plane. The higher the compressive strain of the AlGaN layer, the higher the Al composition at which polarization switching occurs. See T. K. Sharma et al., Phys. Rev. B 84, 035305 (2011); and J. E. Northrup et al., Appl. Phys. Left. 100, 021101 (2012). For growth of QWs compressively strained to AlN, the optical polarization switches to perpendicular to the QW plane at Al compositions greater than 0.80. See R. G. Banal et al., Phys. Rev. B 79, 121308 (2009); and T. K. Sharma et al., Phys. Rev. B 84, 035305 (2011). The Al composition where polarization switching occurs becomes lower as the AlGaN is relaxed so, in general, compressively strained AlGaN QWs are preferred for high light extraction. See J. E. Northrup et al., Appl. Phys. Lett. 100, 021101 (2012).
Quantum size effects, such as changes in QW thickness also affect optical polarization, although this is a less explored area. At a fixed strain, and at Al compositions (Al≧0.25) where the crystal-field split-off valence band is close enough in energy to the ground state to play a role, the optical polarization switches from parallel to perpendicular to the QW plane as the QW thickness increases. See R. G. Banal et al., Phys. Rev. B 79, 121308 (2009); T. K. Sharma et al., Phys. Rev. B 84, 035305 (2011); A. A. Yamaguchi, Phys. Status Solidi C 5, 2364 (2008); and T. M. Al Tahtamouni et al., Appl. Phys. Lett. 101, 042103 (2012). The QW thickness where this switching occurs shifts with strain, increasing in QW thickness with increased compressive strain. See T. M. Al Tahtamouni et al., Appl. Phys. Lett. 101, 042103 (2012). The reports of QW thickness dependence have been limited to theoretical or photoluminescence studies thus far, and at very high Al compositions. See A. A. Yamaguchi, Phys. Status Solidi C 5, 2364 (2008); T. M. Al Tahtamouni et al., Appl. Phys. Lett. 101, 042103 (2012); and H. M. Lu et al., Opt. Express 20, 27384 (2012). An inclusive experimental and theoretical study covering the effect of QW thickness on the optical polarization of electrically-injected UVLEDs has not yet been reported.
Carrier density can also play a role in the optical polarization. The influence of carrier density on polarization switching has been shown theoretically at high Al compositions (Al˜0.7-0.9). See S. H. Park and J. I. Shim, Appl. Phys. Lett. 102, 221109 (2013). Such a trend, although not highlighted, was also observed in a previous theoretical work for lower compositions (Al˜0.39). See S. Wieczorek et al., Appl. Phys. Lett. 84, 4899 (2004). Experimental evidence of carrier density influencing optical polarization has yet been shown.
However, a need remains for an ultraviolet light-emitting diode wherein the composition, thickness, strain, and carrier density of the AlGaN quantum wells are tailored to enable high extraction efficiency.